Strangely enough, when I go into BIOS the USB shows up twice but booting the first one brings up the installer (haven't tried the second).

UEFI boards do that. If you boot the one without a note/prefix/postfix saying its efi or uefi then you boot the installer in legacy mode. And if you have a brand new laptop it will be uefi only.

Make sure you boot the install stick in UEFI mode.

The easiest way is to use the installer during partitioning, and to write a new GPT to the drive. That makes sure the old stuff is deleted. There is nothing in the bios that could do that.

Then you first create a 1024MiB large partition, assign it to be ESP, and the rest is done by the installer.

Next you create a whatever you like large partition for /, ext4 as file system and mount point /.

Take a read on the webpage I already mentioned, there is a installtion instruction with pictures. But as far as I know it just describes the auto mode. Manual partitioning isn't mentioned in there. Its a bit arkward.

For my BIOS settings, I already disabled Secure Boot but didn't mention it in my first post. The "Absolute" tracking system is to my understanding a feature that helps you get the physical location of your PC if you lost it. I didn't have the relevant software so I saw no reason to keep it enabled.

Now in terms of the EFI/UEFI/legacy stuff, I am not very familiar with those so I might need some more help. I can confirm BIOS is listing my USB as UEFI. My BIOS doesn't have a CSM option. I suppose my PC is UEFI only.

Last thing: to my understanding I should be all clear to fully wipe the drive and try again? Theres an option to do that right in BIOS I just dont want to mess anything up. Also, should I go for manual instead of auto partitioning? One of you mentioned making a 1024 MB partition for ESP.

From your description, I'm pretty sure you're using one of the installer ISOs, not the live ISO, but just want confirmation of that.

You mentioned you're using USB for the installer.  How did you copy the ISO image to the USB drive?  The preferred method is to write the ISO directly to the USB block device (for example, using dd).  Multiboot schemes may cause issues with the installer, such as booting it in legacy mode instead of EFI mode.

What is your goal? Replace Win11 completely or a parallel installation to Win11? The steps to take are different. Before going into details, we should clarify this topic.

TPM isn't relevant for the installation of Excalibur.

Personally, I would go and switch off Secure Boot. On some PCs this is called "Other OS". And I delete all SB keys. Don't need them.

What is this "the "Absolute" tracking system"?

This is Audio stuff, you can ignore it until Excalibur is up and running.

You may install w/o network. I often do.

Setting up wifi can be very challening in the installer, depending on the HW used. The better solution: Ethernet normally is no big problem, except for on some exotic chipsets.

Sounds like there is an issue with getting the efi grub installed. That can have several reasons. Maybe a confusion between legacy/efi install?

Some remarks:
Your PC must be an modern UEFI capable system, that means only an installation in efi mode makes sense. Make sure that you have disabled CSM in the bios.

When booting the install media, make sure you boot it in efi mode. The bios should show a prefix like efi or uefi before the name of the device.

For an efi installation you need a FAT32 formatted ESP partition. When you do manual partitioning, the ESP has to be configured in the partitioner as ESP, it does not need to be partitioned. Win leaves a relatively small ESP of a bit more than 100MB. When you do a clean Devuan only install, spend 1024 MiB. 

It should be no deal to use /dev/nvme0n1p2 as / with ext4 file system. When its empty, you can have it formatted by the installer without data loss. Just the assignment has to be correct.

Good luck.

PS: Here you can find some reading: https://www.devuan.org/os/install

you just need make sure that your eventual Windows installation and Linux are installed the same method (UEFI or legacy BIOS), otherwise grub won't see them vis-a-vis.

if you have a newer computer, it will most certainly be with UEFI (even UEFI only);
sometimes Microsoft forces on certain Windows pre-installed OEM-PC Secure-Boot setting, which makes it ab it more difficult to install Linux, but eventually you will succeed.

Configuring shim-signed:amd64
Unable to configure GRUB
Executing 'update-grub' failed
This is a fatal error

After which the installer took me back to the main menu. I had the option to try installing the GRUB boot loader again, but it once again failed. I aborted the install and tried again, but when my PC restarted it took me to some kind of GRUB terminal. I restarted and went to BIOS this time. 'debian' was now above my installation USB in the boot order, but I booted from the installation media manually.

And that's where I'm at now. Partitioning fails every time I try to install. I tried switching the installation USB for a fresh one; no dice. This is the error I keep getting now:

The attempt to mount a file system with type ext4 in /dev/nvme0n1p2 at / failed.

Currently, nvme0n1p2 is a 989GB ext4 partition. So yeah, my computer is a bit unusable right now but at least I can boot from USBs. I could really use some help to fix this and properly install Devuan. I read somewhere that the issue could be that my ext4 partition is corrupted, but I don't know if that is the case for me.